Medical techniques for the treatment of malignant neoplasms in patients often involve the use of radiation. A radiation source, such as a medical linear accelerator, is typically used to generate and direct radiation onto a target area of the patient's body. When applied in the proper doses (dosimetry), the radiation produces an ionizing effect on the malignant tissue, thereby killing the malignant cells without causing significant detrimental effect to the surrounding healthy tissue.
The operational characteristics and output levels of medical linear accelerators are varied. The most common type of accelerator is one that produces pulsed radiation that is output as a rectangular beam with a cross-sectional area typically ranging between 16 to 1600 square centimeters. Continuous (non-pulsed) medical devices, such as a cobalt machine, are also used as a radiation source for treating malignant neoplasms. Other accelerators exist that utilize a swept electron beam modality. These machines sweep a very narrow electron beam across the treatment field by means of varying electromagnetic fields.
All linear accelerators used for the treatment of malignant neoplasms must be calibrated. By this process, a determination is made of how much radiation, in terms of Greys, is produced for each monitor unit displayed on the machine console. The American Association of Physicists in Medicine (AAPM) has established protocols (TG-21 and TG-25) for the required correlation procedures for both electron and photon radiation. There are currently no known radiation measurement systems capable of producing, as an output, calibration data that is in compliance with AAPM protocols. Instead, the measurement data produced by known systems must first be modified by the proper calibration units in order to perform machine calibrations.
The intensity and duration of the radiation treatment must be carefully calculated and administered to produce optimized therapeutic results with attendant patient safety. If too much radiation is administered, the radiation's curative effects may be overwhelmed by its destructive effects to the tissue surrounding the malignancy. If an insufficient amount of radiation is delivered, a tumorcidal dose may not be achieved. Therefore, it is important to know how much radiation will be produced per monitor unit by a particular machine and how that radiation will be distributed within the patient's body.
To accurately determine the intensity and duration (dosage) of radiation received by the patient, a pattern, or map, of the radiation at varying positions within the patient's body must be produced. These patterns, often referred to as profiles, isodose lines or depth dose curves, depending on the type of presentation, are used to model the distribution of radiation inside the patient undergoing the external beam radiation treatment. The resultant data is then evaluated by a qualified medical physicist to ascertain the machine's suitability for use in dosimetry. The data that makes up the pattern is also used by clinical personnel or a treatment planning system computer to determine the machine on time or monitor units required for the prescribed treatment.
Existing systems for measuring radiation produced by medical linear accelerators employ a tank filled with water with a radiation detector immersed within the water. The composition of the human body closely approximates that of water, so the tank (water phantom) provides a good medium for simulating the distribution and intensity of radiation within the patient's body. Radiation produced by the linear accelerator is directed at the water in the phantom tank where the intensity of the radiation at varying depths and positions within the water is measured with the radiation detector. Scattered radiation produced as the primary radiation penetrates the water, as well as direct or primary radiation, is detected by an ion chamber detector, which is essentially an open air capacitor, producing an electrical current corresponding to the number of ions produced within its volume.
A common technique for processing the ion chamber output is to integrate the signal over a fixed period of time. The detector is lowered to a measurement point within the phantom tank where measurements are taken over a time period. The detector is then moved to another measurement point and measurements are taken as the detector is held in position. For each measurement, a statistically significant number of samples is required, so the detector must be held stationary at the measurement position until the required number of readings have been taken. When measuring radiation produced by machines employing an electron swept beam, the detector must remain in position until a sufficient amount of current has been stored.
The signal integration technique typically employs two detectors--a signal channel and a reference channel. This technique chooses a time interval and then presumes that a statistically significant signal will be output by the detector during that time interval. In other words, the signal integration technique does not take into account the fact that the radiation source may be producing pulsed radiation. For a system employing a pulsed radiation source, a statistically significant signal cannot be assured since the measurement period can begin at any time during the pulse train. Moreover, the signal integration technique cannot account for pulses that are dropped to maintain output, as in the case of Varian accelerators with the Dose Rate Servo activated.
Another known technique for processing the ion chamber detector output is a voltage plotting technique. Electrical current output by the detector is converted to a corresponding voltage signal and plotted as a function of the position of the detector within the phantom tank. Each channel must be independently balanced or signal saturation may occur. The resultant data must be further processed by mathematical smoothing techniques before it is considered useable.
Thus, there is a need for a system that can measure the distribution and intensity of radiation produced by medical linear accelerators as the detector is continuously swept through the phantom tank with no mathematical data smoothing required. The system should exhibit the same stability as the accelerator energy output and maintain precise time synchronization with pulsed linear accelerators. Additionally, the system should be capable of providing the necessary functions to calibrate the accelerator based on AAPM Protocol 21 (for photon measurement) and Protocol 25 (for electron measurement).